1. Field of the Invention
This invention relates to an end facet light emitting type light emitting diode (LED) array suitably used in a light source of an electrophotographic printer, a manufacturing method for an end facet light emitting type light emitting device, including an LED frabricated array by use of a dicing technique, and to a measuring method for measuring light emitting characteristics of such a light emitting device.
2. Description of Related Art
Light emitting diode arrays (LED arrays) are used, for example, as a light source for electrophotographic printers. One particular type of such LED arrays is referred as an end facet light emitting type LED array. Each end facet light emitting type LED array has a plurality of light emitting diodes. A known light emitting diode is formed with its light emitting end facet being be perpendicular to the surface of a substrate. Structures and manufacturing methods for the end facet light emitting type LED array are disclosed, for example, in Japanese laid-open Patent Publication No. 2-125,765 (Reference 1) or No. 5-31955 (Reference 2), respectively. For example, Reference 1 discloses that each of the end facet light emitting type LED arrays can be obtained by dicing a semiconductor wafer having stacked layers of an n-type electrode, an n-type GaAs buffer layer, an n-type AlGaAs layer, a p-type AlGaAs layer and a p-type electrode. Also, Reference 2 discloses that a light emitting end facet can be fabricated on a semiconductor wafer having a double hetero structure by employing a dry etching method utilizing a chloride gas. Various types of similar structures having light emitting end facets perpendicular to the surface of a substrate are also disclosed. For example, Japanese laid-open Patent Publication No. 1-117,073 discloses an LED having an inclined rear end facet, in which the inclined rear end effectively reflects unnecessary light to avoid unwanted oscillation. Japanese laid-open Patent Publication No. 3-201,574 also discloses another conventional LED having a severred end facet at the current block layer for preventing light from coming back to an active layer from which the light is emitted.
However, in the case of the conventional LED array in which the light emitting end facet is perpendicular to the surface of the substrate, the size of an area from which the emitting light is taken out or derived (hereinafter called a light deriving area) is limited, at the maximum, by the product of a thickness and a width of the light emitting layer. More concretely, since the p-type AlGaAs layer of the LED array disclosed in Reference 1 or the active layer region, among the n-type clad layer, the active layer and the p-type layer which constitutes the double hetero structure, of the LED array disclosed in Reference 2 corresponds to the light emitting layer, the light deriving area size of such LED arrays is limited by the thickness and the width of the light emitting layer.
To separate each LED array from the others, semiconductor wafers on which LED arrays have been formed have to be precisely cut. One of known methods to cut the wafer is to dice it at the border of each array. To increase the number per wafer of LED arrays to be produced, each array has to be formed without unnecessary space on the wafer. An updated design of such an LED array places the border of each array very close to its light emitting region. When diced, however, such a wafer tends to be cracked, and when the light emitting region of the LED is cracked, the LED cannot properly produce light from the cracked region. Moreover, with a conventional dicing technique, interconnection layers on the top of the LED array are also subject to cutting when the wafer is diced. Burrs that incidentally formed at an interconnection layer when the wafer had been diced may contact with a semiconductor layer having the opposite conductivity to the interconnection layer, thereby short-circuiting between the interconnection layer and the semiconductor layer.
With another conventional method, an end facet of the LED is formed by dry etching method. However, since deep grooves have to be formed at the respective borders of LED arrays to separate one LED array from the others, portions to be eliminated by the dry etching can be widen as time elapses, thereby requiring its array design to be added with extra margins on the limited space on the wafer and thereby reducing the number per wafer of the LED arrays. Japanese laid-open Patent Publication No. 5-69,592 discloses an LED array having a plurality of LEDs, each of which has an inclined light reflecting end facet. In manufacturing this LED array, however, the semiconductor substrate of the LED array is etched by ion milling, so that this method is unsuitable for a design in which each array is formed within a very small area. Furthermore, such ion milling over the tilted substrate may cause damages on an active layer of the LED.
With conventional methods, light emitting characteristics of the LED array could not be measured before each array is cut separately. Accordingly, the light emitting characteristics of the LED array must be measured piece by piece, so that setting the respective arrays to a measurement apparatus inevitably increases overall work load.